Vehicles that incorporate hybrid hydrostatic transmissions, such as earth moving machinery, industrial equipment, and others are typically referred to as having a series hybrid arrangement or a parallel hybrid arrangement. In the series hybrid arrangement, at least one accumulator and a control valve are used to collect energy from or deliver stored energy to a conventional hydrostatic transmission, which includes a first hydrostatic unit drivingly engaged with a power source and a second hydrostatic unit drivingly engaged with a transmission output. In the parallel hybrid arrangement, at least one accumulator and a control valve are used to collect energy from or deliver stored energy to a hydrostatic unit, which is drivingly engaged with both a power source and a transmission output.
Vehicles which include such hybrid hydrostatic transmissions are typically operated in a working cycle mode and a transport cycle mode. In the working cycle mode, the vehicle typically operates at lower speeds and with a variable gear ratio. Torque interruptions within the hybrid hydrostatic transmissions are strongly disfavored in the working cycle mode. The working cycle mode is typically employed in lifting operations and pushing operations. In the transport cycle mode, the vehicle typically operates at higher speeds and with one or more fixed gear ratios. The transport cycle mode is typically employed in hauling operations or while moving the vehicle between areas the working cycle mode is used in. Increased efficiency of the hybrid hydrostatic transmissions is strongly desired in both the working cycle mode and the transport cycle mode.
Vehicles which include such hybrid hydrostatic transmissions are also subject to torque interruptions during a shifting procedure between the working cycle mode and the transport cycle mode. Such torque interruptions may result in reduced performance of the vehicle and operator dissatisfaction.
It would be advantageous to develop a hybrid hydrostatic driveline and a method of operation thereof for a vehicle that provides the benefits of a series hybrid arrangement and a parallel hybrid arrangement, reduces torque interruptions during operation and shifting procedures, and increases an efficiency of a vehicle the hybrid hydrostatic driveline is incorporated in.